Hot melt adhesive (HMA), also called hot glue, is a type of thermoplastic adhesive which is commonly sold as solid cylindrical sticks of varied diameters made to be used using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, that the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed from the heated nozzle is initially hot enough to burn and even blister skin. The glue is tacky when hot, and solidifies in a few seconds to one minute. Hot melt adhesives may also be applied by dipping or spraying.
In industrial use, hot melt adhesives provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, as well as the drying or curing step is eliminated. Hot melt adhesives have long life expectancy and in most cases could be discarded without special precautions. A few of the disadvantages involve thermal load from the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, approximately complete melting from the adhesive. This is often reduced by utilizing Flame laminating machine that after solidifying undergoes further curing e.g., by moisture (e.g., reactive urethanes and silicones), or perhaps is cured by ultraviolet radiation. Some HMAs might not be immune to chemical attacks and weathering. HMAs tend not to lose thickness during solidifying; solvent-based adhesives may lose as much as 50-70% of layer thickness during drying.
Hot melt glues usually contain one base material with some other additives. The composition is usually formulated to have a glass transition temperature (start of brittleness) beneath the lowest service temperature along with a suitably high melt temperature as well. The degree of crystallization ought to be up to possible but within limits of allowed shrinkage. The melt viscosity and also the crystallization rate (and corresponding open time) could be tailored for that application. Faster crystallization rate usually implies higher bond strength. To achieve the properties of semicrystalline polymers, amorphous polymers would require molecular weights too high and, therefore, unreasonably high melt viscosity; the usage of amorphous polymers in hot melt adhesives is usually only as modifiers. Some polymers can form hydrogen bonds between their chains, forming pseudo-cross-links which strengthen the polymer.
The natures from the polymer and also the additives utilized to increase tackiness (called tackifiers) influence the character of mutual molecular interaction and interaction with all the substrate. In just one common system, EVA is utilized as the main polymer, with terpene-phenol resin (TPR) because the tackifier. The two components display acid-base interactions between the carbonyl teams of vinyl acetate and hydroxyl sets of TPR, complexes are formed between phenolic rings of TPR and hydroxyl groups on the surface of aluminium substrates, and interactions between carbonyl groups and silanol groups on surfaces of glass substrates are formed. Polar groups, hydroxyls and amine groups can form acid-base and hydrogen bonds with polar groups on substrates like paper or wood or natural fibers. Nonpolar polyolefin chains interact well with nonpolar substrates.
Good wetting in the substrate is important for forming a satisfying bond in between the Automatic conveyor belt cutting machine and also the substrate. More polar compositions usually have better adhesion due to their higher surface energy. Amorphous adhesives deform easily, tending to dissipate most of mechanical strain within their structure, passing only small loads on the adhesive-substrate interface; a relatively weak nonpolar-nonpolar surface interaction can form a relatively strong bond prone primarily to some cohesive failure. The distribution of molecular weights and degree of crystallinity influences the width of melting temperature range. Polymers with crystalline nature tend to be more rigid and also have higher cohesive strength compared to the corresponding amorphous ones, but additionally transfer more strain to the adhesive-substrate interface. Higher molecular weight in the polymer chains provides higher tensile strength and also heat resistance. Presence of unsaturated bonds makes pqrpif adhesive more vunerable to autoxidation and UV degradation and necessitates use of antioxidants and stabilizers.
The adhesives are often clear or translucent, colorless, straw-colored, tan, or amber. Pigmented versions can also be made and even versions with glittery sparkles. Materials containing polar groups, aromatic systems, and double and triple bonds tend to appear darker than non-polar fully saturated substances; whenever a water-clear appearance is desired, suitable polymers and additives, e.g. hydrogenated tackifying resins, need to be used.
Increase of bond strength and service temperature can be accomplished by formation of cross-links in the polymer after solidification. This can be achieved by utilizing polymers undergoing curing with residual moisture (e.g., reactive polyurethanes, silicones), contact with ultraviolet radiation, electron irradiation, or by other methods.
Effectiveness against water and solvents is critical in certain applications. For example, in PU Leather/PVC Bronzing Machine, potential to deal with dry cleaning solvents may be needed. Permeability to gases and water vapor might or might not be desirable. Non-toxicity of the base materials and additives and absence of odors is very important for food packaging.